What is the Barlow formula?

The Barlow formula is P = (2 × t × Y) / OD, where P is the burst pressure in psi, t is the nominal wall thickness in inches, Y is the minimum yield strength of the pipe material in psi, and OD is the outer diameter in inches. It is the industry-standard method for estimating the internal pressure at which a pipe will yield.

Does this give the API-rated burst pressure?

The Barlow formula gives the theoretical yield-based burst pressure. API 5C3 ratings apply a 0.875 wall-thickness tolerance factor and an 80% design factor, which results in a burst rating roughly 87.5% of the value calculated here. This tool uses the nominal wall and full yield strength; apply API derating factors as required by your design standard.

What does the safety factor mean?

Safety factor = burst pressure rating / actual test or operating pressure. A value above 1.0 means the casing can withstand the test pressure. Well engineering standards typically require a minimum safety factor of 1.0 to 1.25 for burst during the design phase. Always confirm against your operator's or regulatory requirements.

What is the difference between minimum and specified yield strength?

Minimum yield strength (Y) is the lowest value guaranteed by the API grade specification, used for conservative design calculations. Actual material yield may be higher. API grades like J-55 and L-80 are named after their minimum yield strength in ksi (1,000 psi).

What API grades are commonly used?

Common API casing grades and their minimum yield strengths are: J-55 (55,000 psi), K-55 (55,000 psi), L-80 (80,000 psi), N-80 (80,000 psi), C-90 (90,000 psi), T-95 (95,000 psi), P-110 (110,000 psi), and Q-125 (125,000 psi). Higher grades are used for deeper wells or higher-pressure environments.

Why does inner diameter matter?

Inner diameter (ID = OD − 2t) determines the flow area through the casing and the maximum outer diameter of tools, tubing, and completion equipment that can pass through. A thicker wall increases burst rating but reduces ID, so the trade-off must be checked against completion and production requirements.

Casing Burst Pressure Calculator

Name: Casing Burst Pressure Calculator
Availability: InStock
Author: Nham Vu

Calculate the internal burst pressure rating of oil and gas well casing using the Barlow formula, and compute the safety factor against a test pressure.

Casing Parameters

Outer Diameter — OD (inches)

Common sizes: 4.5, 5.5, 7, 9.625, 13.375 in.

Wall Thickness — t (inches)

Nominal wall; must be less than OD/2.

API Casing Grade

Test / Operating Pressure (psi) (optional)

Enables safety factor calculation.

Enter casing OD, wall thickness, and yield strength to compute the Barlow burst pressure rating.

Burst Pressure Rating

—

psi (Barlow formula)

Inner Diameter

—

inches (OD − 2t)

Safety Factor

—

Enter test pressure to compute

Calculation Detail

Outer diameter (OD) —

Wall thickness (t) —

Min. yield strength (Y) —

Inner diameter (ID = OD − 2t) —

Burst pressure (2tY / OD) —

API Casing Grade Reference

Grade	Min Yield (psi)	Typical Use
J-55 / K-55	55,000	Shallow wells, surface casing
L-80 / N-80	80,000	Intermediate depths, H2S service (L-80)
C-90	90,000	Sour gas environments
T-95	95,000	Deep sour wells
P-110	110,000	Deep wells, high-pressure zones
Q-125	125,000	Ultra-deep, HPHT wells

Source: API Spec 5CT. Yield values shown are minimum; actual material may be higher. API burst ratings apply additional derating factors not included in this Barlow calculation.

Summary